Heat treatment for cold worked nitinol to impart a shape setting capability without eventually developing stress-induced martensite

Abstract

Cold worked nickel-titanium alloys that have linear pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device having a strut formed body deployed from a sheath. In one application, an embolic protection device that employs a linear pseudoelastic nitinol self-expanding strut assembly with a small profile delivery system for use with interventional procedures. The expandable strut assembly is covered with a filter element and both are compressed into a restraining sheath for delivery to a deployment site downstream and distal to an interventional procedure. Once at the desired site, the restraining sheath is retracted to deploy the embolic protection device, which captures flowing emboli generated during the interventional procedure. Linear pseudoelastic nitinol is used in the medical device as distinct from non-linear pseudoelastic (i.e., superelastic) nitinol.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates generally to application of nickel-titanium alloys to medical devices. More precisely, the present invention is directed to cold worked nickel-titanium alloys that have pseudoelastic behavior without a phase transformation or onset of stress-induced martensite as applied to a medical device deployed from a sheath.[0002]Near equi-atomic binary nickel-titanium alloys (nitinol) are known to exhibit “pseudoelastic” behavior when given certain cold working processes or cold working and heat treatment processes following hot working. Generally speaking, “pseudoelasticity” is the capacity of the nickel-titanium alloy to undergo large elastic strains on the order of 8 percent or more when stressed and to substantially fully recover all strain upon removal of the stress. Substantially full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.[0003]Pseudoelasticity...

AI Technical Summary

Benefits of technology

[0012]During its operation, the linear pseudoelastic nitinol device can be stressed without developing stress-induced martensite in the alloy. Consistent with this behavior, an idealized stress-strain curve of the linear pseudoelastic nitinol does not contain any flat stress plateaus. Furthermore, despite application of stress, the nitinol alloy does not undergo a phase transformation from austenite to martensite or vice versa.

[0013]The resulting preferred embodiment device has greater mechanical strength at any given strain as compared to a device made of a standard superelastic nitinol. The stress-strain curve of the present invention linear pseudoelastic nitinol device also possesses more energy storage capacity. As a result, for a given desired performance requirement, the present invention linear pseudoelastic nitinol device allows for smaller struts and consequently a lower profile useful in crossing narrow lesions.

[0014]Another advantage is that because the present invention uses linear pseudoelastic nitinol, the underlying alloy can be selected from a broader range of available materials yet still maintain consistent, mechanical properties. In other words, there is less sensitivity to material variations and processing vagaries as compared to superelastic nitinol. In addition, since the linear pseudoelastic nitinol has no transformation from martensite to austenite or vice versa, there is less of an influence by temperature-related effects.

[0015]There are many specific applications for the present invention including vena cava filters, septal plugs, just to name a few. One specific application for the present invention is in a filtering device and system for capturing embolic debris in a blood vessel created during the performance of a therapeutic interventional procedure, such as a balloon angioplasty or stenting procedure, in order to prevent the embolic debris from blocking blood vessels downstream from the interventional site. The devices and systems of the present invention are particularly useful while performing an interventional procedure in critical arteries, such as the carotid arteries, in which vital downstream blood vessels can easily become blocked with embolic debris, including the main blood vessels leading to the brain. When used in carotid procedures, the present invention minimizes the potential for a stroke occurring during the procedure. As a result, the present invention provides the physician with a higher degree of confidence that embolic debris is being properly collected and removed from the patient's vasculature during the interventional procedure.

Problems solved by technology

Non-linear pseudoelasticity results from cold working and subsequent heat treatment.

Such an occlusion can cause devastating consequences to the patient.

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